Rotary printing machines, such as rotary offset lithographic machines, for example, utilize printing cylinders to which printing plates are conformed and secured. The printing plates rotate with the cylinders and apply the desired impression to a rubber covered blanket cylinder, which then transfers the impression to a sheet or web passed between the blanket cylinder and an opposing roll or plate. Typically, means are provided on the printing cylinder for mechanically engaging one or both ends of the printing plate in order to keep the plate properly positioned on the cylinder. Commonly, this is done by providing means for clamping the ends of the printing plate. In some cases, adhesive is utilized, alone or in conjunction with other means, to hold the printing plates in position.
It has been proposed heretofor to utilize vacuum means to retain the printing plates in position on the printing cylinders. Representative of prior proposals for this purpose are the Johnson et al U.S. Pat. No. 2,060,082, the Lake U.S. Pat. No. 3,112,698, the Musgrave U.S. Pat. No. 3,380,381, and the Demaria et al U.S. Pat. No. 3,463,476. The theoretical advantages of the vacuum cylinder are evident, in that it is expected to be able to more easily mount printing plates thereon. Changing of printing plates is also expected to be facilitated. However, notwithstanding the expected advantages to be derived from the vacuum cylinder, its commercial acceptance has been extremely limited, because of the practical difficulties experienced in achieving an effective design of a vaccum cylinder suitable for use under conditions of day to day commercial production.
As its basic objective, the present invention seeks to provide a novel and improved design and construction of a vacuum-operated printing cylinder suitable for utilization under conditions of typical commercial production and which does not require the use, in conjunction with the vaccum holding means, of mechanical or other auxiliary holding means to assist in securing the printing plate on the face of the cylinder.
Principal structural elements of the new vacuum-operated printing cylinder include (a) an elongated supporting shaft extending from one end to the other of the cylinder and sufficiently beyond to provide bearing support, (b) a plurality of cylinder supporting rings secured in axially spaced relation on the bearing shaft, and (c) a hollow cylinder sleeve secured to the supporting rings and forming therewith one or more (advantageously a plurality) of vacuum chambers within the printing cylinder. The supporting shaft and rings are provided with internal passages for the evacuation of the internal chambers. To particular advantage, independent valve means are provided for selectively evacuating all or less than all of the plurality of chambers of the vacuum cylinder, so that small size printing plates may be readily utilized and/or so that printing plates may be removed and replaced from part of the cylinder surface, without disturbing other plates secured by vacuum in adjacent areas.
In the improved vacuum-operated printing cylinder of the invention, an effective balance is provided with respect to the amount and distribution of exposed vacuum area on the outer surface of the cylinder, such that adequately distributed vacuum holding power is available to secure the printing plates, while at the same time minimizing leakage, which would reduce the overall holding effectiveness of the cylinder. In some forms of the new printing cylinders, vacuum communication is provided by appropriately distributed small radial bores. In other forms, discontinuous annular grooves are provided. And in still other forms, means are provided for diffusing the vacuum over the surface of the cylinder.
Pursuant to the invention, certain constructions of the new vacuum cylinder utilize a plurality of independent vacuum chambers. Such structures advantageously utilize a combination of internal piping and bored passages in the supporting shaft, to provide communication between the vacuum source and the respective individual chambers. In addition, each such individual chamber is provided with a manually accessible valve arrangement, enabling the chamber to be isolated from the vacuum source where desired. Thus, if only a portion of the surface of cylinder is to be covered by the printing plate, the otherwise exposed portions of the cylinder surface may be isolated from the vacuum source to prevent excessive leakage.
In part, the invention is directed to improved and advantageous structural features for a vacuum-operated printing cylinder, which simplify and improve its construction. One such feature relates to the provision of a plurality of axially spaced support rings, each of slightly increasing diameter from one end to the other of the cylinder assembly. The outer cylinder sleeve member, which is supported by the rings, is in turn provided with shouldered bearing areas of progressively increasing diameter. The arrangement is such that the outer cylinder sleeve may be applied axially over the pre-assembled shaft and support rings, achieving a force fit therewith only over the last inch or so of axial assembly. Thus, the final assembly of the principal elements is relatively convenient and straightforward. At the same time, however, the airtight integrity of the various vacuum chambers is easily maintained.
In accordance with one of the more specific aspects of the invention, in the form of the new vacuum-operated cylinder in which vacuum distribution is effected by means of annular grooves in the cylinder, an improved and simplified arrangement is provided for blocking off or dead ending the grooves to avoid leakage adjacent the ends of a vacuum-held printing plate. In this respect, after first forming continuous annular grooves in the printing cylinder, the cylinder is provided with a longitudinally extending surface groove, which is continuous from one end to the other of the cylinder. A soft metal strip is then tightly received in the longitudinal groove, to effectively block off all of the annular grooves and provide a predetermined circumferential discontinuity between adjacent ends of each groove. While it is known in the prior art to provide means for individually dead ending the annular grooves (e.g. Lake U.S. Pat. No. 3,112,698), the arrangement of the invention is considered to be more simplified and versatile than the prior art proposals.
In accordance with other specific aspects of the invention, the vacuum-operated printing cylinders, otherwise as described above, may additionally be provided with vacuum diffusion means over the outer surface areas, in the form of porous filter media or sintered, porous surface coatings. The use of porous filter media is particularly advantegeous for some applications, in that coverings of filter paper may be provided in various thicknesses, to compensate for different thicknessess of the printing plates which may be utilized by the printer. When using vacuum diffusion means, less consideration need be given to minimizing or avoiding leakage beyond the edges of the printing plate, because the diffusing mediums easily limits such leakage to amounts within the capacity of vacuum pumps typically used for the purpose.
For a better understanding of the above and other features and advantages of the invention, reference should be made to the following detailed description and the accompanying drawing.